Monika
Małgorzata
Tomczyk
Desenho de materiais funcionais 2D para futuras
aplicações em microeletrónica
Designing 2D functional materials for future
microelectronics applications
Monika
Małgorzata
Tomczyk
Desenho de materiais funcionais 2D para futuras
aplicações em microeletrónica
Designing 2D functional materials for future
microelectronics applications
Tese apresentada à Universidade de Aveiro para cumprimento dos requisitos necessários à obtenção do grau de Doutor em Ciência e Engenharia de Materiais, realizada sob a orientação científica da Doutora Paula M. L. S. Vilarinho, Professora Associada do Departamento de Engenharia de Materiais e Cerâmica da Universidade de Aveiro.
Thesis presented to the University of Aveiro in fulfillment of the requirements for the awarding of the degree of Doctor in Materials Science and Engineering under the scientific guidance of Professor Paula M. L. S. Vilarinho, Associate Professor of the Department of Materials and Ceramic Engineering of the University of Aveiro.
Apoio financeiro da FCT e do FSE no âmbito do III Quadro Comunitário de Apoio.
o júri / the jury
presidente Prof. Doutora Ana Isabel Couto Neto da Silva Miranda
professora catedrática da Universidade de Aveiro
vogais / examiners committee Prof. Doutora Maria de Jesus Matos Gomes
professora catedrática, Escola de Ciências, Universidade do Minho
Prof. Doutor Abílio de Jesus Monteiro Almeida
professor associado com agregação, Faculdade de Ciências, Universidade do Porto
Prof. Doutor Luís Manuel Cadillon Martins Costa
professor associado com agregação, Universidade de Aveiro
Prof. Doutora Isabel Maria das Mercês Ferreira
professora associada, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa
Prof. Doutora Paula Maria Lousada Silveirinha Vilarinho
agradecimentos / acknowledgements
The majority of experimental work was performed at the Department of Materials and Ceramic Engineering, part of the associated laboratory – CICECO (Centre for Research in Ceramics and Composite Materials), and I am mostly grateful to my supervisor Prof. Paula M. Vilarinho for giving me the opportunity to do PhD under her supervision. Her constant encouragement, guidance and excellent mentorship over the past four years helped me to develop, organize, write and present good scientific communications, ending up with this thesis.
Part of the work was carried out in The Instituto de Ciencia de Materiales de Madrid (ICMM), Consejo Superior de Investigaciones Cientificas (CSIC) in Madrid (Spain) in the group of Electroceramics for Information Technologies. I would like to thank Prof. M. Lourdes Calzada, Dr. Ricardo Jiménez and Dr. Iñigo Bretos for for sharing their extensive knowledge on low temperature thin films preparation and electrical characterization.
I would like to thank Prof. Ian M. Reaney from Department of Engineering Materials, University of Sheffield for consultations about the microstructure of the investigated thin films via TEM.
I would like to thank Dr. Daniel G. Stroppa from International Iberian Nanotechnology Laboratory for his kind help on TEM sample preparation by FIB and HRTEM studies.
I would also like to thank all the lab members for their help over the years. Then, plenty of others have had impact on the work, and I want to thank all of them.
I acknowledge FCT, the Portuguese Foundation for Science and Technology, for financial support, under the grant SFRH/BD/80123/2011.
palavras-chave Eletrocerâmicas, Filmes finos, Ferroelétricos sem chumbo, BiFeO3, (Na0.5Bi0.5)TiO3, deposição de soluções químicas, Eletrónica flexível, Silício, poliimida, método de precursores de soles difásicos (SDSG), método de foto-deposição de solução química (FDSQ)
resumo Devido à redução de dimensões e ao aumento da velocidade de processamento de dados nos dispositivos microeletrónicos baseados em semicondutores convencionais, estão a ser exploradas abordagens inovadoras envolvendo novos materiais tais como óxidos funcionais. Com o rápido desenvolvimento da indústria eletrónica existe uma maior necessidade de elevado desempenho, de elevada fiabilidade, e de componentes eletrónicos miniaturizados integrados em vários dispositivos. A fim de tornar os dispositivos amplamente acessíveis e de fácil utilização, requisitos adicionais devem ser considerados: o tamanho e peso desejados, o custo reduzido, o baixo consumo de energia e a portabilidade. Materiais funcionais de baixa dimensionalidade são muito promissores para cumprir essas exigências. Em particular, os ferroeléctricos de filmes finos bidimensionais (2D) têm recebido grande atenção devido à sua crescente utilização em memórias não voláteis, detectores piroelétricos, transdutores piezoeléctricos miniaturizados e dispositivos sintonizáveis de micro-ondas. A temperatura de cristalização é um parâmetro chave na preparação de ferroelétricos 2D. Muitos filmes finos ferroelétricos são cristalizados a temperaturas >600 °C. Esses valores estão acima da temperatura que certos elementos do dispositivo funcional podem suportar. Recentemente, este facto tornou-se ainda mais importante, devido às promissoras aplicações que podem ser consideradas caso os ferroeléctricos 2D sejam compatíveis com substratos poliméricos flexíveis de baixo custo e de baixo ponto de fusão. A compatibilidade de filmes finos ferróicos com estes últimos tipos de substratos é muito difícil, mas se conseguida pode ampliar acentuadamente a gama de aplicações para os mais recentes requisitos de eletrónica flexível e microeletrónica, onde dispositivos leves e baratos são exigidos.
Neste trabalho, é implementada uma combinação da modificação da química de precursores e assistência por luz UV, com promoção simultânea da cristalização pela introdução de sementes nanocristalinas na solução precursora, para a fabricação de filmes finos ferróicos sem chumbo - Método de Precursores Fotossensíveis Semeados. Neste contexto, o principal objetivo deste trabalho foi fabricar filmes finos sem chumbo BiFeO3 (BFO) e Na0.5Bi0.5TiO3 (NBT) a baixas temperaturas (~300 °C) com uma resposta ferroelétrica competitiva. Além disso, a investigação do efeito do elétrodo-base sobre as propriedades dielétricas e ferroelétricas de filmes finos de BFO foi levada a cabo, e a comparação entre o comportamento de condensadores de BFO com base em IrO2, LaNiO3 (LNO) e Pt foi estabelecida. Adicionalmente, os efeitos dos vários eléctrodos sobre a microestrutura de filmes finos ferroeléctricos de BFO foram estudados por microscopia eletrónica de transmissão (TEM) de alta resolução. Primeiramente, filmes finos finos de perovesquite BFO e NBT foram preparados sobre substratos de silício revestidos com Pt, por deposição de solução química. Os filmes finos de BFO foram preparados a temperaturas na gama de
400-de fuga nos filmes finos diminuem com a diminuição da temperatura 400-de processamento, a polarização de filmes finos de BFO preparados com excesso Bi e recozidos a 400 e 450 °C pode ser efetivamente comutada à temperatura ambiente. Obtiveram-se valores de polarização remanescente de Pr ~10 e
~60 μC/cm2 com campos coercivos de E
C ~ 205 e 235 kV/cm para os filmes finos
preparados a 400 e 450 °C, respectivamente. Os filmes finos de NBT foram preparados a temperaturas entre 400 e 650 °C. As propriedades estruturais e ferroelétricas dos filmes foram examinadas. A constante dieléctrica observada e as perdas dieléctricas a 100 kHz são 616 e 0,032, respectivamente, enquanto que a polarização remanescente observada e o campo coercivo são Pr ~
24 μC/cm2 e E
C ~ 215 kV/cm, respectivamente para o filme de NBT recozido a
650 °C. O recozimento térmico, em atmosfera de oxigénio após cada camada de revestimento, é eficaz na promoção da cristalização do filme na fase de perovesquite romboédrica a uma baixa temperatura de 400 °C. No entanto, obteve-se um ciclo P-E quase linear para os filmes NBT cristalizados a 400 °C devido à sua incipiente cristalinidade.
Os filmes finos de BFO foram depositados numa gama de elétrodos para determinar o seu papel no controlo da formação de fases e da microestrutura. A cristalização em elétrodos de óxido seguiu a sequência: amorfa → Bi2O2(CO3) → perovesquite, enquanto que nos elétrodos de Pt cristalizaram diretamente a partir da fase amorfa. Os elétrodos de IrO2 promoveram a formação da fase de perovesquite à temperatura mais baixa e o LNO induziu adicionalmente o crescimento epitaxial local. O LNO tem a estrutura de perovesquite com o parâmetro de rede a = 0.384 nm, compatível com o de BFO, a = 0.396 nm, e assim a epitaxia é mais provável. Todas as composições exibiram precipitados inteiramente coerentes ricos em Fe dentro do interior de grão da matriz de perovesquite, enquanto que a incoerente segunda fase de Bi2Fe4O9 foi também observada nos limites de grão de BFO crescido em eléctrodos de Pt. Esta última pode ser observada por difração de raios X, bem como TEM, mas os precipitados coerentes foram observados apenas por TEM, principalmente evidenciados pelo seu contraste Z em imagens de campo escuro anular. Estes dados têm consequências acentuadas permitindo alargar a utilização de filmes de BFO sob campo aplicado, a aplicações como atuadores, sensores e aplicações de memória.
Em seguida, os filmes finos de BFO foram depositados em substratos de Si com elétrodos distintos, como Pt, LNO e IrO2, para investigar o efeito do elétrodo-base sobre o crescimento e as propriedades elétricas do BFO. Todas os filmes de BFO são compostos por grãos colunares cujo tamanho é dependente do elétrodo-base. Não se observou textura para filmes de 320 nm de espessura fabricados em Pt orientado (111). Os filmes sobre eléctrodos de óxido, em particular sobre LNO são altamente orientados no plano (012). A grande polarização remanescente em BFO/Pt e BFO/IrO2 é atribuída à alta contribuição de corrente de fuga. Os filmes BFO de 400 nm de espessura em LNO possuem uma baixa densidade de corrente de fuga ~4 × 10-6 A/cm2, uma grande polarização remanescente de 50 μC/cm2 e um pequeno campo coercitivo de 180 kV/cm à temperatura ambiente. Demonstramos que as camadas de LNO aumentam a cristalinidade e a orientação de filmes finos BFO, o que se reflete nas suas propriedades funcionais. Este estudo mostra que, além da simples necessidade de filmes monofásicos, os elétrodos de óxido de metal têm um impacto relevante no desenvolvimento de filmes finos BFO de alta qualidade fabricados por métodos químicos de deposição de solução. Estes resultados têm uma implicação grande para a fabricação de dispositivos BFO baseados em filmes finos.
Finalmente, provamos que é possível fabricar diretamente filmes finos de BFO sem chumbo em substratos flexíveis de poliamida com funcionalidades ferroelétricas e magnéticas (multiferroicidade) à temperatura ambiente. O nosso método inovador, baseado em soluções de Precursores Fotossensíveis e nanosementes cristalinas, foi usado com sucesso para diminuir a temperatura
keywords Electroceramics, Thin films, Lead-free ferroelectric, BiFeO3, Na0.5Bi0.5TiO3, chemical solution deposition, flexible electronics, Si, polyimide, seeded precursor solution method, photochemical solution deposition method
abstract With the dimensions reduction and data processing speeds increasing of conventional semiconductor based microelectronic devices, innovative approaches involving new materials such as functional oxides are being explored. With the rapid development of the electronics industry there is a need for high performance, high reliability and miniaturized electronic components integrated into various devices. In order to make the devices user friendly and widely accessible, additional requirements should be considered: the desired size and weight, low cost, low power consumption, and portability in addition to high levels of functionality. Low dimensional functional materials hold great promises to fulfil those requirements. In particular, two-dimensional (2D) thin film ferroelectrics have received wide attention because of their growing use as non-volatile memories, pyroelectric detectors, miniaturized piezoelectric transducers and tunable microwave devices. Crystallization temperature is a key parameter in preparation of 2D-ferroelectrics. Many ferroelectric thin films are crystallized at temperatures >600 °C. This is above the temperature that certain elements of the functional device can withstand. Recently it became even more important due to promising applications that can be envisaged if 2D-ferroelectrics will be compatible with low cost, low melting temperature flexible polymeric substrates. The compatibility of ferroic thin films with those last types of substrates can markedly widen the range of applications towards the most recent requirements of flexible electronics and microelectronics, where lightweight and cheap devices are demanded.
In this work, a combination of the modification of precursor chemistry and the assistance of UV-light, with simultaneous promotion of crystallization by introducing nanocrystalline seeds in the precursor solution, is implemented to fabricate lead-free ferroic thin films - Seeded Photosensitive Precursor Method. Within this context, the main objective of this work was to fabricate lead-free BiFeO3 (BFO) and Na0.5Bi0.5TiO3 (NBT) thin films with a competitive ferroelectric response at low temperatures. Moreover, investigations of the effect of the bottom electrode on the dielectric and ferroelectric properties of BFO thin films was conducted and the comparison between the behavior of IrO2, LaNiO3 (LNO) and Pt based BFO capacitors established. Additionally, the effects of these various bottom electrodes on the microstructure of BiFeO3 ferroelectric films was studied by high-resolution TEM.
Firstly, BFO and NBT perovskite thin films were prepared on Pt-coated silicon substrates by chemical solution deposition. BFO was prepared at temperatures in the range 400-500 °C, and from stoichiometric and Bi excess precursor solutions. Crystalline BFO films were obtained at the lowest temperature limit of 400 °C. The films prepared with Bi excess possess more defined ferroelectric hysteresis loops than those without any excess; for films with thicknesses ~150 nm. As the leakage current densities in the films decrease with decreasing
from 400 to 650 °C. Structural and ferroelectric properties of the films were examined. The observed dielectric constant and dielectric losses at 100 kHz are 616 and 0.032, respectively, while the observed remanent polarization and coercive field are Pr ~ 24 μC/cm2 and EC ~ 215 kV/cm, respectively for the NBT
film annealed at 650 °C. Thermal annealing in an oxygen atmosphere after each layer of coating is effective in promoting crystallization of the film into rhombohedral perovskite phase at a low temperature of 400 °C. However, almost linear, P-E loop was obtained for those NBT films crystallized at 400 °C due to incipient crystallinity.
BFO thin films were grown on a range of electrodes to determine their role in controlling phase formation and microstructure. The crystallization on oxide electrodes followed the sequence: amorphous → Bi2O2(CO3) → perovskite, while those on Pt crystallized directly from the amorphous phase. IrO2 electrodes promoted perovskite phase formation at the lowest temperature and LaNiO3 additionally induced local epitaxial growth. LNO has the perovskite structure with lattice parameter a = 0.384 nm, compatible with that of BFO, a = 0.396 nm and thus epitaxy is more likely. It was observed for the first time that all compositions exhibited fully coherent Fe-rich precipitates within the grain interior of the perovskite matrix, whereas incoherent Bi2Fe4O9 second phase was also observed at the grain boundaries of BFO grown on Pt electrodes. The latter could be observed by X-ray diffraction as well as transmission electron microscopy (TEM) but coherent precipitates were only observed by TEM, principally evidenced by their Z contrast in annular dark field images. These data have pronounced consequences for the extended use of BFO films under applied field for actuator, sensor and memory applications.
Then, BFO thin films were deposited on Si-based substrates with distinct electrodes, such as Pt, LNO, and IrO2, in order to investigate the effect of bottom electrode on the growth and electrical properties of BFO. All BFO films are composed of columnar grains which size is dependent on the bottom electrode. No texture was observed for 320 nm thick films fabricated on (111) oriented Pt. Films on oxide electrodes, in particular on LNO are highly (012) oriented. The large remanent polarization in BFO/Pt and BFO/IrO2 is attributed to the high leakage current contribution. 400 nm thick BFO films on LNO possess a low leakage current density ~4 × 10-6 A/cm2, a large remanent polarization of 50 µC/cm2 and a small coercive field of 180 kV/cm at room temperature. We demonstrate that LNO layers enhance the crystallinity and orientation of BFO thin films, which is reflected in their functional properties. This study shows that besides the simple need of monophasic films metal oxide electrodes have a relevant impact on the development of high quality BFO thin films fabricated by chemical solution deposition methods. These results have a broad implication for the fabrication of BFOthin film based devices.
Finally, we prove that it is possible to directly fabricate lead-free BFO thin films on flexible polyamide substrates with ferroelectric and magnetic functionalites (multiferroicity) at room temperature. Our own proprietary novel solution-based Seeded Photosensitive Precursor Method was successfully used to decrease the crystallization temperature of BFO thin films down to a temperature as low as 300 °C, the lowest reported up to now for the preparation of multiferroic BFO thin films. Despite this exceptionally low thermal budget a remanent polarization Pr
of 2.8 µC/cm2 is obtained for the seeded + UV films, with a coercive field E
C of
300 kV/cm. The synthesis strategy based on the use of seeded photosensitive precursors can be transferred to any family of functional metal oxide.
Table of contents
Table of contents ... XV
List of Figures ... XIX
List of Tables ... XXVII
List of Symbols ... XXVIII
List of Abbreviations ... XXX
Chapter 1.
INTRODUCTION ... 1
1.1. Motivation ... 2
1.2. Organization of the thesis ... 6
Chapter 2.
STATE OF THE ART ... 7
2.1. Introduction to functional materials: properties and applications ... 8
2.2. Lead free ferroelectrics... 16
2.2.1. Bismuth ferrite – BiFeO3 ... 18
2.2.1.1. The structures of bismuth ferrite ... 19
2.2.1.2. Physical properties of BiFeO3 ... 22
2.2.1.3. Device applications ... 28
2.2.1.3.1. Ferroelectric random access memories ... 28
2.2.1.3.2. Magnetoelectric and multiferroic memories ... 30
2.2.1.4. Processing of BFO ... 32
2.2.1.4.1. BFO single crystals ... 32
2.2.1.4.2. BFO bulk ceramics ... 34
2.2.1.4.3. BFO epitaxial thin films ... 38
2.2.2. Sodium bismuth titanate – Na0.5Bi0.5TiO3 ... 46
2.2.2.1. Structure of NBT ... 46
2.2.2.2. Physical properties of Na0.5Bi0.5TiO3 ... 48
2.2.2.1. Processing of NBT ... 50
2.3. Low temperature preparation of thin films ... 54
2.3.1. Modifications at the “precursor/green state film level” ... 54
2.3.2. Modifications at the “post deposition processing level” ... 57
2.3.3. Photo Chemical Solution Deposition ... 59
2.3.4. Seeded Diphasic Sol-Gel ... 61
2.3.5. Seeded Photosensitive Precursor Method... 62
2.4. Summary ... 64
Chapter 3.
EXPERIMENTAL METHODS ... 67
3.1. Experimental procedures ... 68
3.1.1. Chemical solution process ... 68
3.1.1.1. Precursor solutions preparation ... 69
3.1.1.2. Thin films deposition – spin-coating ... 73
3.1.2. Hydrothermal preparation of seeds... 74
3.2. Characterization techniques ... 77
3.2.1. Thermal analyses ... 77
3.2.2. X-ray Diffraction ... 78
3.2.2.1. Stress analysis by XRD ... 80
3.2.3. Electron Microscopy... 81
3.2.3.1. Scanning Electron Microscopy ... 81
3.2.3.3. Scanning Transmission Electron Microscopy ... 84
3.2.3.4. TEM sample preparation ... 84
3.2.4. Raman Spectroscopy ... 86
3.2.5. Electrical measurements ... 87
3.2.6. UV-Vis spectroscopy ... 89
3.2.7. Scanning Probe Microscopy ... 90
3.2.8. Other methods ... 91
Chapter 4.
RESULTS AND DISCUSSION ... 93
4.1. Na0.5Bi0.5TiO3 thin films – an effect of annealing temperature on microstructure and electrical performance ... 94
4.1.1. Experimental methods ... 96
4.1.2. Results and discussion ... 97
4.2. BiFeO3 thin films on Si: Bi-excess and temperature effect ... 106
4.2.1. Experimental methods ... 107
4.2.2. Structure and microstructure ... 108
4.2.3. Electrical characterization ... 112
4.3. Bottom electrode effect on the microstructure of BiFeO3 thin films ... 117
4.3.1. Experimental methods ... 120
4.3.2. Results and discussion ... 121
4.4. Correlation of electrodes with orientation and electrical performance of BiFeO3 thin films ... 135
4.4.1. Experimental methods ... 140
4.4.2. Results ... 142
4.5. Direct fabrication of BiFeO3 thin films on polyimide substrates for flexible
electronics ... 159
4.5.1. Experimental procedure ... 163
4.5.2. Results and discussion ... 166
4.5.2.1. Characterization of seeds ... 166
4.5.2.2. Characterization of thin films ... 169
Chapter 5.
SUMMARY ... 178
Scientific output ... 182
Doctoral Programme... 184
List of Figures
Figure 1-1. Schematic showing the motivation of the work. ... 5 Figure 2-2. Spectrum of functional ceramic materials applications. Adopted from [30]. ... 9
Figure 2-3. Perovskite structure with chemical formula ABO3. The red spheres are oxygen
atoms, the blue spheres are B-atoms (a smaller metal cation, such as Ti4+), and the green
spheres are the A-atoms (a larger metal cation, such as Ca2+). ... 10
Figure 2-4. Schematic representation of piezoelectricity, pyroelectricity and ferroelectricity based on crystal symmetry and their origin. Scheme also shows that all ferroelectric materials are pyroelectric and piezoelectric but not vice versa [35]. ... 12 Figure 2-5. Some of the examples of applications of ferroelectric materials due to their piezoelectric, dielectric and pyroelectric properties [38]. (Thin Film PZT for Semiconductor Application Trends & Technology 2013 Report by Yole Developpement). IPD – Integrated Passives Devices, HDD – Hard Disk Drive, MEMS - MicroElectroMechanical System, FeRAM - Ferroelectric Random Access Memory, IR – Infrared. ... 13 Figure 2-6. Thin film deposition techniques. Adapted from Haertling [40]. ... 14
Figure 2-7. Relationship between the Curie Temperature (TC) and remanent polarization (Pr)
for selected groups of lead-based and lead-free ferroelectric materials (with selected
references embedded) [40, 51-73]. In particular, BFO possesses high TC and Pr, thus it can
be implemented in devices working at high temperatures. ... 17 Figure 2-8. The relationship between multiferroic and magnetoelectric materials [76]. .... 18 Figure 2-9. BFO crystal structure representations: (a) hexagonal unit cell (six formula units), (b) rhombohedral unit cell (two formula units), and (c) pseudo-cubic unit cell (one formula unit). Red balls represent the Bi ions, blue ones the Fe ions and green the O ions. Yellow/blue/green arrows symbolize a shift of the Bi/Fe cations and the octahedral rotation, respectively. (d) Unit vectors and unit cell in the hexagonal (black), rhombohedral (red), and pseudocubic (blue) notations [82]. ... 20 Figure 2-10. Scheme of the antiferromagnetic structure of BFO, the magnetic moments describe a cycloid with a period of 64 nm [89]. ... 21
Figure 2-15. Hysteresis loop of Mn-doped BFO and Pb(Zr, Ti)O3 thin films grown on
platinized silicon substrates by chemical solution deposition. The remnant polarization of
BFO is very large, ~100 μC/cm2, along the polar [111] direction - much larger than the
polarization of the most widely used material in ferroelectric memories, as PZT or SBT [115]. ... 29 Figure 2-16. Sketch of a possible MERAM element using BFO [116]; description in the text. ... 31 Figure 2-17. A ferroelectric photovoltaic memory prototype device [118]. (a) Topography
of the device with pre-set polarization direction (polarization up and down). (b) Voc of all
cells measured under 20 mW/cm2 light. ... 32
Figure 2-18. Fernlike dendritic single crystals of BFO (a) group of leaves grown in a Pt crucible; (b) individual leaf [121]. ... 33
Figure 2-19. Phase diagram of the Bi2O3−Fe2O3 system. The α, β, and γ phases are
rhombohedral, orthorhombic, and cubic, respectively [121]... 35
Figure 2-20. Thermodynamic instability of BFO and reaction kinetics in the Bi2O3–Fe2O3
system: a) Calculated temperature dependence of the Gibbs free energy (ΔrG°m) of the
equilibrium reaction between BFO and the Bi- and Fe-rich phases [126]; b) Proposed
reaction pathway mechanism for the solid-state synthesis of BFO from Bi2O3 and
Fe2O3 [127]. ... 36
Figure 2-21. Symmetry-strain dependence for epitaxial BFO thin films. Epitaxial strain imposed by the substrate enables the control of the crystal structure of the grown films: tetragonal, rhombohedral, orthorhombic and monoclinic structures are form in BFO epitaxial thin films as a function of the strain (from [97]). ... 39 Figure 2-22. Schematic representation of the fundamental steps in CSD preparation of ferroelectric oxide thin films [80]. ... 40 Figure 2-23. Reported possible phase transition routes based on TEM and X‐ray/neutron diffraction studies for NBT ceramics [168]. ... 47 Figure 2-24. Crystal structures of NBT: (a) rhombohedral R3c (hexagonal setting) and (b) monoclinic Cc [169]. ... 47
Figure 2-25. Influence of a) Bi and b) Na non-stoichiometry on piezoelectric coefficient d33
and depolarization temperature Td. Na deficiency and Bi excess in the nominal starting
composition enhances the DC resistivity and piezoelectric coefficient d33, but lowers
depolarization temperature Td. Na excess or Bi deficiency decrease the DC resistivity and
d33 but enhances Td [171, 172]. ... 49
Figure 2-26. Arrhenius-type plots of bulk conductivity for nominal compositions NBT, Na0.51Bi0.5TiO2.985 (Na0.51BT), Na0.49Bi0.5TiO3.015 (Na0.49BT), Na0.5Bi0.51TiO2.995 (NBi0.51T)
and Na0.5Bi0.49TiO3.005 (NBi0.49T) [174]. ... 51
Figure 2-27. Flow chart of the Photochemical Solution Deposition technique... 60 Figure 2-28. Flow chart of the Seeded Diphasic Sol-Gel technique. ... 61 Figure 2-29. Flow chart of Seeded Photosensitive Precursor Method that combines PCSD and SDSG techniques. ... 63 Figure 2-30. Ferroelectric hysteresis loop of a PZT film on flexible polyimide with a
thickness of ~190 nm, showing values of Pr ~ 15μC/cm2, from the compensated loop; the
upper inset corresponds to the compensated ferroelectric hysteresis loop, and the lower inset corresponds to the non-switching contribution to the polarization [16]. ... 63 Figure 3-31. Thin films preparation via chemical solution deposition... 69 Figure 3-32. BFO precursor solutions preparation via sol-gel process. Detailed experimental protocol is described in text. ... 70 Figure 3-33. UV-absorption spectrum of BFO precursor solution. The absorption band at ~275 nm corresponds to the π → π* electronic transition of acetylacetonate complexes that activate decomposition of metal precursors and oxidation of organic species while UV-irradiated. ... 71 Figure 3-34. NBT precursor solutions preparation via sol-gel process. Detailed experimental protocol is described in text. ... 72 Figure 3-35. UV-absorption spectrum of NBT precursor solution. The absorption maximum at ~270 nm matches to the π → π* electronic transition developed by the acetylacetonate molecules. ... 72
Figure 3-36. Scheme presenting the protocol for hydrothermal synthesis of BFO nanoparticles. ... 76 Figure 3-37. Scheme of grazing incidence XRD experiment. ... 79 Figure 3-38. Concept of diffraction stress analysis [236]. ... 80 Figure 3-39. Schematic representation of a construction of TEM (http://barrett-group.mcgill.ca/tutorials/nanotechnology/nano02.htm). ... 83 Figure 3-40. Scheme presenting sample preparation by FIB. ... 86 Figure 3-41. Schematic representation of the main components of Raman spectrometer (www.renishaw.com)... 87 Figure 3-42. PFM experimental setup. When AC signal is applied between the tip and the bottom electrode the induced cantilever deflection is measured as an electrical signal by lock-in amplifier (www.csinstruments.eu). ... 91 Figure 4-43. DTA/TG curves for NBT gel powders, at 5 °C/min heating and cooling rates. The conversion of the amorphous precursor into a full crystalline material proceeds in the temperature ≥500 °C. ... 98 Figure 4-44. XRD patterns of NBT thin films annealed at distinct temperatures: 400, 450, 500, 550, 600 and 650 °C, for 60 s. It can be seen that all films, heat treated at temperatures ≥400 °C, are crystalline. ... 99 Figure 4-45.SEM top views and cross sections of NBT films annealed at 400 (a), 450 (b), 500 (c), 550 (d), 600 (e), and 600 °C (f). The microstructure of NBT films is rather smooth formed by equiaxed grains, and there is an evidence of cracking in the case of films annealed at temperatures >500 °C. The thickness of the NBT films was estimated to be ~350 nm.101 Figure 4-46. Hysteresis loops a) and current loops b) of NBT thin films crystallized at different temperatures. The characteristic maxima of the current curves related to the switching of the ferroelectric domains are observed for the films annealed at ≥450 °C. However, almost linear, P-E loop was obtained for those NBT films crystallized at 400 °C due to an incipient crystallinity. ... 103 Figure 4-47. XRD results of BFO thin films prepared from stoichiometric precursor solution
perovskite phase whereas BFO annealed at 500 °C also exhibits a weak, broad peak
attributed to Bi2Fe4O9 (*). ... 110
Figure 4-48. SEM micrographs (top view and cross section) of BFO films annealed at distinct temperatures and deposited from stoichiometric or Bi excess precursor solutions. The average thickness of the films is ~200 nm, not observing noticeable dissimilarities in the cross-section microstructure images. As the annealing temperature increases, a large amount of grains seems to grow at the expense of the fine-grained phase. ... 111 Figure 4-49. Polarization and current density hysteresis loops, traced at 1 kHz measured at different temperatures. BFO films crystallized at distinct temperatures and derived from stoichiometric precursor solutions. The characteristic current maxima in J-E loops are related to the switching of the ferroelectric polarization and are observed in all of the films. ... 113 Figure 4-50. Polarization and current density hysteresis loops, traced at 1 kHz measured at different temperatures. BFO films crystallized at distinct temperatures and derived from Bi excess precursor solutions. The one crystallized at 500 °C suffers a dielectric breakdown during switching at temperatures higher than 225 K. ... 114 Figure 4-51. GI-XRD traces of BFO films fabricated by chemical solution deposition on
Pt/Si (BFO/Pt), LNO/Pt/Si (BFO/LNO) and IrO2/Si (BFO/IrO2) and annealed at 500 °C. All
peaks associated with BFO/IrO2 and BFO/LNOmay be attributed either to the electrode or
a BFO perovskite phase whereas BFO/Pt also exhibits a weak, broad peak attributed to
Bi2Fe4O9(*). ... 122
Figure 4-52. GI-XRD patterns of BFO thin films annealed at 500°C for different times using
heating rates of 30 and 100 °C/s. (a) BFO/Pt, (b) BFO/LNO, and (c) BFO/IrO2. * denotes
Bi2O2(CO3). ... 124
Figure 4-53. Bright-field TEM images of cross-sections of a) BFO/Pt, b) BFO/LNO and c)
BFO/IrO2 thin films showing a columnar grain structure. The average width of the columnar
grains is 80, 140 and 200 nm for BFO/LNO, BFO/Pt and BFO/IrO2, respectively. ... 125
Figure 4-54. ADF TEM cross-sectional micrographs of the (a) BFO/Pt, (b) BFO/LNO and
(c) BFO/IrO2 thin films. A grain boundary with second phase (indicated by the arrowhead)
indicating that these regions are poor in heavier elements compared with the bright contrast in neighboring areas... 127 Figure 4-55. (a) STEM micrographs of second phase at the grain boundary of a BFO/Pt thin film with inset line-scan EDS plots obtained along the white line. The grain boundary phase is poor in Bi and rich in Fe with respect to the matrix, (b) high resolution lattice image of a
second phase, grain boundary particle whose d-spacings are consistent with d110 (0.579 nm)
Bi2Fe4O9 phase. ... 128
Figure 4-56. (a) HRTEM micrograph of the inclusion in the grain interior which indicates that they are coherent and (b) proposed mechanism of inclusion formation... 130
Figure 4-57. ADF TEM micrographs of (a) BFO/Pt, (b) BFO/LNO and (c) BFO/IrO2
interfaces. No secondary phase was detected at the interface between BFO and bottom electrodes. ... 131
Figure 4-58. Schematic illustrating the average residual stresses calculated by XRD sin2ψ
method for BFO films on distinct electrodes. The lattice and thermal expansion coefficient of the BFO and bottom electrode materials were listed [269, 302, 303]. The stresses are
tensile in all cases. The highest residual stress was observed for BFO/IrO2 and the lowest in
BFO/LNO thin films. ... 132 Figure 4-59. Dark-field image of a columnar grain in the BFO film on LNO. Local epitaxial growth of BFO on LNO was observed. ... 133
Figure 4-60. Schematic representation of BFO/Pt, BFO/IrO2 and BFO/LNO thin films
structures. ... 141
Figure 4-61. XRD patterns of BFO thin films on Pt/Si, LNO/Si and IrO2/Si, with (a) 20L
(320 nm) and (b) 30L (400 nm), and annealed at 500 °C. Diffraction lines of crystallographic
planes of R3c space group are marked. All peaks associated with BFO/Pt, BFO/IrO2 and
BFO/LNOmay be attributed to either the electrode or the BFO perovskite phase. ... 143
Figure 4-62. XRD pole figure maps corresponding to (012) diffraction reflection of BFO
thin films on Pt/Si, LNO/Si and IrO2/Si, with 20L (a) and 30L (b). Rocking curves are also
presented (c). Strong (012) texture is observed for BFO/IrO2-20L and BFO/LNO
Figure 4-63. Raman spectra of BFO thin films on Pt/Si, LNO/Si and IrO2/Si, with 20 (solid
lines) and 30 (dashed lines) deposition layers. ... 146 Figure 4-64. Plan view and cross section view (inset) SEM micrographs of BFO/Pt-20L (a),
BFO/Pt-30L (b), BFO/LNO-20L (c), BFO/LNO-30L (d), BFO/IrO2-20L (e) and BFO/IrO2
-30L (f). The estimated thickness of films was ~320 nm for BFO/Pt-20L, BFO/IrO2-20L and
BFO/LNO-20L, while for BFO/Pt-30L, BFO/IrO2-30L and BFO/LNO-30L it is ~400 nm.
The results show that the average grain sizes increase with increasing thickness, except for the BFO/LNO-30L. ... 148 Figure 4-65. The polarization (P-E) hysteresis loops (top) and the switching currents versus
applied field (I-E) (bottom), for BFO/Pt, BFO/LNO and BFO/IrO2 measured at room
temperature and at 4 kHz for films with thickness of 320 nm (solid line) and 400 cm (dashed line). A significant contribution from the leakage current was obvious, as evidenced by the
round shape of the P-E loops for BFO/Pt, BFO/IrO2 and BFO/LNO-20L... 149
Figure 4-66. The leakage current density (J) versus electric field (E) of BFO thin films on different electrodes having thickness of (a) 320 nm (20L) and (b) 400 nm (30L). For films with LNO used as a bottom electrode, the leakage current density was substantially reduced
to 5.28 × 10-7 and 4.04 × 10-6 A·cm-2 for BFO/LNO-20L and BFO/LNO-30L consistent with
saturated hysteresis curves... 150
Figure 4-67. Remanent polarization, Pr, as a function of the BFO single crystals, bulk
ceramics and thin films fabrication temperature. Clear regression of the BFO ferroelectric properties is observed for fabrication temperatures lower than 400 °C. ... 163 Figure 4-68. Schematic representation of the strategy that combines seeded diphasic precursors and photochemical solution deposition. This method consists in the preparation of activated sols, deposition of the precursor films, followed by photoexcitation/pyrolysis and final crystallization [16]. ... 165 Figure 4-69. XRD pattern (a), TEM bright field image (b) and EDS spectra (c) of BFO nanoseeds synthesized by the hydrothermal process. BFO nanoparticles present a single-phase perovskite structure and ultra-fine spherical morphology with the average particle size of about 30 nm. ... 168
Figure 4-70. (a) Photograph of a flexible BFO film fabricated on polyamide substrate. (b) XRD patterns of BFO thin films processed at 300 °C on flexible substrates. Unseeded films are amorphous. Reflections corresponding to the perovskite phase are detected for seeded and seeded + UV BFO films. ... 170 Figure 4-71. SEM micrographs of seeded (a) and seeded +UV (b) of BFO thin films on flexible polyamide substrate. The morphology of the films consists of nanosized grains with a mean diameter of ~70 and ~100 nm for seeded and seeded + UV BFO thin films, respectively. ... 171 Figure 4-72. The polarization (P-E) hysteresis loops (a) and the switching currents versus applied field (b), for seeded and seeded and UV-irradiated BFO films measured at 140 K
and 10 kHz. A remanent polarization Pr of 2.8 µC/cm2 is obtained for the seeded + UV film,
with a coercive field EC of 300 kV/cm confirm the ferroelectric nature of the perovskite
phase prepared at only 300 °C. ... 174 Figure 4-73. Topographic and VPFM amplitude and phase images of seeded and seeded + UV BFO thin films on flexible polyimide substrates crystalized at 300 °C for 120 min. Both films depict obvious piezoelectric behaviour, however areas with poor contrast, indicated by blue dashed line, likely correspond to regions of the films with incipient crystallization and domains with in-plane polarization. Some of the domains are highlighted in green for comparison. ... 175 Figure 4-74. Magnetic hysteresis of seeded and seeded + UV BFO at 300 K up to 10 kOe. BFO on flexible polyimide substrates depict the weak ferromagnetic response. The magnetic response increases with decreasing grain size of BFO thin films. ... 176
List of Tables
Table 2-1. Selected processing parameters for typical CSD routes used for BFO thin films fabrication. ... 41 Table 2-2. Summary of ferroelectric properties of pure BFO single crystals, bulk ceramics and thin films. ... 45 Table 2-3. Electrical properties of reported NBT thin films [191, 192, 194, 195]. ... 53 Table 2-4. Some of the structural, physical and chemical characteristics of NBT [50] and BFO [54, 55], as well as PZT [52]. ... 65 Table 3-5. Experimental details of BFO and NBT precursor solutions preparation by solution methods. ... 73 Table 4-6. Properties of NBT thin films prepared at this work and reported in literature. 104 Table 4-7. Results derived from the current density loops measured at 200 K and 1 kHz for BFO films annealed at distinct temperatures and deposited from stoichiometric or Bi excess precursor solutions. ... 115 Table 4-8. Electrical properties of BFO thin films on different bottom electrodes / substrates prepared by CSD and other related methods for comparison. ... 139
Table 4-9. Some physical and crystallographic parameters of Pt, IrO2, LNO and BFO. .. 151
Table 4-10. Comparison of functional properties of BFO thin films on different electrodes
fabricated by CSD. Large ferroelectricity in BFO/Pt-20L and BFO/IrO2 (20L and 30L) is
List of Symbols
ΔrG°m Gibbs free energy of the equilibrium reaction
a lattice parameter
A absorbance
Å angstrom, unit of length
c lattice parameter
Cp capacitance of the parallel circuit
d, dn, dψ space between the planes (lattice spacing)
d31 piezoelectric transverse coefficient
d33 piezoelectric coefficient in the z-direction
dn
dψ lattice spacing for each ψ
dE/dH magnetoelectric coefficient
E electric field
EC coercive electric field
f frequency
I current
I, I0 transmitted/incident radiation
J current density
k31, k33 electromechanical coupling coefficients
n refractive index
ν Poisson’s ratio
P polarization
Pr remanent polarization
Ps spontaneous polarization
Rp resistance of the parallel circuit
S area
t time
t film thickness
T transmittance
tanδ dielectric loss
TC Curie temperature
Tm melting temperature
TN Neel temperature
V voltage
Z atomic number
α thermal expansion coefficient
σ residual stress
γ pyroelectric coefficient
εr dielectric constant
ε0 dielectric constant of vacuum, ε0 = 8.854 × 10−12 F⋅m−1
θ, ω incident angle
List of Abbreviations
2D two-dimensional
ABO3 perovskite structure
AC alternating current
AcOH acetic acid, C2H4O2
ADF annular dark-field
AFE antiferroelectric state
AFM antiferromagnetic
AFM atomic force microscopy
BET Brunauer–Emmett–Teller
BF bright field
BFO bismuth ferrite, BiFeO3
BiOAc bismuth acetate, Bi(CH3COO)3
BLNF La and Nb co-doped BiFeO3
BSE backscattered electrons
BST barium strontium titanate, (Ba, Sr)TiO3
BT barium titanate, BaTiO3
CMOS complementary metal oxide semiconductor
CSD chemical solution deposition
CVD chemical vapour deposition
DC direct current
DF dark field
DTA differential thermal analysis
EBSD diffracted backscattered electrons
EDS energy-dispersive X-ray spectrometry
EELS electron energy-loss spectrometry e.g. for example (Latin exemplī grātiā) et al. and others (Latin et alii)
etc. and other things (Latin etceteros) EtOH ethanol, C2H6O
FE ferroelectric
Fe(acac)3 iron(III) acetylacetonate, Fe(C5H7O2)3
FIB focus ion milling
FM ferromagnetic
FTIR Fourier transformed infrared Hacac acetylacetone, C5H8O2
HAADF high-angle annular dark-field GI-XRD grazing incidence X-ray diffraction
IC integrated circuit
i.e. it is (Latin id est)
ITRS International Technology Roadmap for Semiconductors JCPDS Joint Committee on Powder Diffraction Standards
IR infrared
KNN potassium sodium niobate, (K, Na)NbO3
LDFZ laser-diode-heated floating zone
LNO lanthanum nickelate, LaNiO3
LPCVD low-pressure chemical vapour deposition MEMS microelectromechanical systems
MERAM magnetoelectric random access memory MOCVD metalorganic chemical vapour deposition
MOD metalorganic decomposition
MPB morphotropic phase boundary
NaOAc sodium acetate, CH3COONa
NBT sodium bismuth titanate, Na0.5Bi0.5TiO3
P-E polarization-electric field
pc pseudo-cubic
PCSD photo chemical solution deposition
PFM piezoresponse force microscopy
PLD pulsed laser deposition
PT lead titanate, PbTiO3
PVA poly(vinyl alcohol)
PVD physical vapour deposition
PZT lead zirconate titanate, PbZr1-xTixO3
RF radio frequency
RoHS Restriction of Hazardous Substances (Directive 2002/95/EC)
RT room temperature
SAD selected area diffraction
SBT strontium bismuth tantalate, SrBi2Ta2O9
SDSG seeded diphasic sol gel
SEM scanning electron microscopy
SPM scanning probe microscopy
SPS spark plasma sintering
SRO strontium ruthenate, SrRuO3
STEM scanning transmission electron microscopy t-OBu tert-butoxide, (CH3)3CO
TEM transmission electron microscopy
TF thin film
TG thermogravimetric analysis
Ti(OiPr)4 titanium tetra-isopropoxide, C12H28O4Ti
TMR tunnel magnetoresistance
UV ultraviolet
WEEE Waste of electrical and electronic equipment (Directive 2012/19/EU)
VSM vibrating sample magnetometer
XPS X-ray photoelectron spectroscopy
1.1. Motivation
With the dimension’s reduction and speeds increasing of conventional semiconductor based microelectronic devices, scientific community and the technology sector are facing difficulties in keeping pace with the newer device demands of the modern times. Innovative approaches involving new materials such as functional oxides are being explored to challenge this problem. With this rapid development of the electronics industry there is a need for high performance, high reliability and miniaturized electronic components integrated into various devices. In order to make the devices user friendly and widely accessible, additional requirements should be considered: the desired size and weight, low cost, low power consumption, and portability [1]. Low dimensional ferroelectric materials hold great promises to fulfil those requirements. In particular, two-dimensional (2D) thin film ferroelectrics have received wide attention because of their growing use as non-volatile memories, pyroelectric detectors, miniaturized piezoelectric transducers and tunable microwave devices [2]. Utilization of novel materials and structures in real microelectronics applications requires integration of ferroelectric thin films on silicon with the conventional platform, such as CMOS (Complementary Metal-Oxide Semiconductor) technology. Among the many methods available for processing of ferroelectric thin films chemical solution deposition (CSD) is possibly the one with the lowest capital cost, while offering high versatility for creation of a wide range of compositions, microstructures and ultimately properties. Therefore, CSD is one of the most frequently used preparation methods for the fabrication of 2D-ferroelectrics. Crystallization temperature is a key parameter in preparation of 2D-ferroelectrics. Many ferroelectric thin films are crystallized at temperatures >600 °C. This is above the temperature that certain elements of the functional device can withstand. Recently it became even more important due to promising applications that can be envisaged if 2D-ferroelectrics will be compatible with low cost, low melting temperature flexible and rigid metallic and polymeric substrates.
For several years, low temperature synthesis of ferroelectric-films has been attempted with modifications at the “precursor/green state film level” and at the “post depositions level”. Within the first, use of seed-layers and of excess of volatile components or combination of both is widely reported [3-6]. Control of solution chemistry to increase
homogeneity at the molecular level and thus, reactivity of precursors, has been used for the preparation of ferroelectric-films at low-temperatures as well [7]. Depending on the composition 2D ferroelectrics have been reported at around 500 °C, but the main drawback are the very poor properties, if any.
Concerning modifications at the “post deposition processing level” the most widely used strategy involves rapid thermal annealing (RTA). While RTA of lead-based perovskite films can both minimize the formation of the intermediate phases and greatly reduce thermal budget needed for crystallization, required process temperatures are still too high for some applications [8]. In the meantime other alternative methods such as laser-assisted crystallization [8] or laser lift-off [9], are being used for 2D-ferroelectrics.
Worthwhile to mention are attempts associated with the use of diphasic precursor sols (SDSG) process, developed by Paula Vilarinho research group, that allowed the preparation
of lead zirconate titanate (Pb(Zr, Ti)O3, PZT) films at 410 °C/30 h and 550 °C/30 min with
very reasonable dielectric response [10-13]. In this methodology perovskite nanometric particles are dispersed in precursor solution and act as seeds to promote nucleation of the perovskite phase at low temperatures. Crystallization kinetics is enhanced and the activation energy for the perovskite formation is reduced, microstructure improved and better dielectric response obtained. Another important approach is the use of Photo-Chemical-Solution Deposition (PCSD), developed by Lourdes Calzada research group, based on the use of sol-gel precursors sensitive to ultraviolet (UV) light [14] and on the use of UV radiation sources of high intensity [15] to catalyze precursors chemical reactions towards oxide crystallization. The combination of nucleation of crystalline phase at low-temperatures, by modification of the precursors chemistry, with simultaneous promotion of the crystallization, by introducing nanocrystalline nucleus, was recently used for fabrication of PZT thin films at low synthesis-temperature with optimized dielectric response [16].
The development of less hazardous compounds is considered a crucial challenge nowadays, especially in the electronic industry, where legislations enforce alternative non-hazardous materials [17]. Therefore, in the case of ferroelectrics, relaxor-ferroelectrics and piezoelectrics, strong efforts are directed on looking for lead free alternatives for the commercially used PZT. Bismuth-based perovskites, such as those based on the multiferroic
(Na0.5Bi0.5)TiO3 (NBT), are shown as promising candidates in this area [18, 19]. Limitations
for these compounds are related to the high annealing temperatures necessary for the crystallization of these perovskite phases and volatilization of high-vapour pressure elements such as Bi at the temperatures required for the film fabrication.
Bismuth ferrite has undoubtedly been studied more extensively than any other multiferroic being hailed the most promising lead free single phase multiferroic candidate for devices, due to its multiferroic nature above room temperature [20]. However, dielectric losses, leakage current and tendency to fatigue of BFO are the main hurdles to overcome for any kind of electronic applications [20]. The majority of these devices are fabricated such
that the ferroic is deposited on a bottom electrode, typically using Pt/TiO2/SiO2/Si substrates
to provide an electrical contact for input and output data. Naturally, the performance of the device is strongly dependent on the quality of the interface between the ferroic and electrode/substrate heterostructure. Degradation in performance is commonly attributed to interfacial effects that induce internal stresses, modify the local defect chemistry and create non-optimized crystallographic orientations [21]. In principle, these limitations may be minimized if a suitable interlayer is introduced between the film and the Pt electrode.
Moreover, oxide electrodes, such as SrRuO3, IrO2, RuO2 and LaNiO3 (LNO) used instead of
Pt improve performance since they prevent undesired compositional modification and impede charge interdiffusion processes [22]. However, up to now, very few reports have been available on the dielectric and ferroelectric behavior of chemical solution deposited
BFO thin films on LNO and IrO2 electrodes [23-27] and no research focusing on the
comparative study of oxide electrodes and Pt has been conducted.
Therefore, the main aim of this PhD work is to fabricate functional lead free piezoelectric and ferroelectric BFO and NBT thin films with optimized ferroelectric properties, at low temperatures (<450 °C) by a modified CSD method, to be compatible with the silicon based technology and principally with flexible substrates, such as polymeric, with technological added value to be used in future generation of microelectronic devices. The reduction of the synthesis temperature will be attained by a process that results from the combination of two unique synthesis processes, SDSG with PCSD, the so-called Seeded Photosensitive Precursor Method.
Growth of a technologically important and lead free multifunctional material, BFO is chosen to demonstrate the versatility of this method to grow complex oxide thin films directly on flexible substrates at temperature as low as 300 °C. The evaluation of the ferroelectric and piezoelectric properties of these low-temperature BFO films will be presented.
Moreover, investigation of the effect of the bottom electrode on the dielectric and ferroelectric properties of BFO thin films will be conducted. The comparison between the
behavior of IrO2, LNO and Pt based BFO capacitors will be established. Additionally, the
effects of these various bottom electrodes on the microstructure of BFO ferroelectric films will be studied by high-resolution TEM and annular dark field (ADF) scanning transmission electron microscopy (STEM) in order to establish the microstructure-processing parameters relationship is principal to yield high quality BFO thin films.
The main objective of this PhD work is presented in Figure 1-1.
1.2. Organization of the thesis
The thesis is organized as follows:
The present chapter (Chapter 1) describes the motivation and the objective of the work. Chapter 2 provides the state of the art of the materials under investigation in the thesis. The importance of functional materials, with particular incidence on complex oxides with
perovskite-like structure, ABO3, is presented. The chapter includes a detailed review of
structural, physical and chemical features of lead free ferroics as BFO and NBT, with published works on solution processed thin films. Different strategies to decrease the crystallization temperature of ferroelectric thin films are reviewed.
Chapter 3 presents the experimental methods used and implemented in the thesis: processing procedures and characterization techniques.
Chapter 4 is fully devoted to the disclosure and discussion of the achieved results. Within this chapter, five subsections correspond to the following topics: 4.1 – NBT thin films by CSD annealed at different temperatures, 4.2 – BFO thin films preparation by chemical solution deposition towards finding temperature limit of crystallization, 4.3 and 4.4 – comparative study of BFO thin films on distinct bottom electrodes: their microstructure and electrical performance, and 4.5 – strategy towards low temperature processing of BFO on flexible substrates.
And finally, chapter 5 summarizes the main observation and results obtained during the realization of this work, and draws main conclusions, highlighting the main contribution of our work.
Chapter 2.
STATE OF THE ART
Abstract: The purpose of this chapter is to provide the background in which the current work was developed by presenting a review on the state of the art of the most significant aspects related with the thesis subject. Within this context it starts by emphasising the importance of functional oxides in microelectronics. Basic concepts and physical principles of relevant properties of functional materials are introduced with special focused on ferroelectricity and ferroelectric thin films preparation and applications. Some integration issues into devices are presented and the need for low temperature processing of ferroelectric thin films is highlighted. A detailed review
on multiferroic BiFeO3 (BFO) and ferroelectric Na0.5Bi0.5TiO3 (NBT) is
subsequently presented. The literature review covers the available information on the processing, properties and application of these compounds. The chapter further includes details on the major synthesis approaches, used to decrease the crystallization temperature of ferroelectric thin films.
2.1. Introduction to functional materials: properties and
applications
Functional materials are defined as materials that possess particular properties and perform specific functions [28]. The physical and chemical properties are sensitive to a change in the environment such as temperature, pressure, electric field, magnetic field, optical wavelength, adsorbed gas molecules and the pH value. The functional materials utilize the native properties and functions of their own to achieve an intelligent action [29]. Examples include semiconductors ferroelectrics, piezoelectrics, magnetic materials, ionic conductors, etc. Functional materials are found in all classes of materials: ceramics, metals, polymers and organic molecules [28].
Functional ceramic materials consist of a huge group of inorganic compounds, and mainly include piezoelectric, dielectric, ferroelectric, semiconductor, superconducting, magnetic, and currently the very attractive group of multiferroic materials that shows simultaneous ferroelectric order with ferromagnetic ordering. The functional ceramic materials can be classified as [30]:
Electrical/magnetic (for example electroceramics as their primary function is related to the electrical properties), to be used as insulators, semiconductors, conductors and magnets for capacitors, memories, data and information storage, energy conversion,
etc.;
Optical, to be used as components for lenses, lasers, fibers due to their good transparency to light with distinct wavelength;
Chemical, to be used as catalysts, sensors;
Biological, to be used as bioceramics, mostly for implants, and as nanomaterials in drug delivery systems.
Figure 2-2. Spectrum of functional ceramic materials applications. Adopted from [30].
The dominant compositions among functional ceramic materials are oxides that offer a
variety of chemical, physical, structural and microstructural features [31]. ABO3 perovskite
and perovskite related compounds are one of the dominant families of functional oxide ceramics. This is a large family of compounds having crystal structures related to the mineral
perovskite CaTiO3. In the ideal form the crystal structure of a cubic ABO3 perovskite is
described as a corner sharing [BO6] octahedra with the A cation occupying the 12-fold
coordination site formed in the middle of the cube of eight such octahedral (Figure 2-3). The interest in perovskite – type compounds arises from the large variety of properties they exhibit and also and very importantly from the flexibility to accommodate almost all of the elements of the periodic table in the structure what permits designing materials performance adequate to applications; this constitutes a unique relevant added value of this family of
materials [32]. Perovskite materials due to their intriguing and unusual physical properties have been extensively studied for both practical applications and theoretical modelling, and many discoveries from high-temperature superconductors to the organic-inorganic semiconductors for high-efficiency photovoltaics lead to many revolutionary discoveries for new device concepts [33]. Perovskite-type ferroelectrics are indeed of great interest and show a very wide range of useful functional properties [34].
Figure 2-3. Perovskite structure with chemical formula ABO3. The red spheres are
oxygen atoms, the blue spheres are B-atoms (a smaller metal cation, such as Ti4+), and
the green spheres are the A-atoms (a larger metal cation, such as Ca2+).
A ferroelectric perovskite has a permanent spontaneous polarisation PS that can be
switched in direction by the application of an external electric field. Ferroelectricity is a symmetry based phenomena. Low symmetry of a crystal, in particular a non-centrosymmetric crystal symmetry, is essential for the occurrence of ferroelectricity. From the 32 classes of symmetry, 11 point groups possess a centre of symmetry hence, they are nonpolar and 21 possess a non-centrosymmetric structure, hence they are polar or piezoelectric. Form these 21 non centrosymmetric 20 point groups materials show
electric polarisation in these polar materials changes with temperature, making them pyroelectric. Ferroelectrics are a subgroup of pyroelectrics [35].
Ferroelectrics are dielectrics that exhibit spontaneous polarization below the
ferroelectric Curie temperature (TC), and the polarization direction can be changed by an
applied electric field. At temperatures above TC, the crystals are nonpolar (centrosymmetric
crystal structure), and no longer ferroelectric and behave like normal dielectrics [36]. The dielectric constants of ferroelectric materials are extremely high, especially near the Curie temperature. A ferroelectric crystal consists of domains; i.e., regions with uniform spontaneous polarization. The formation of ferroelectric domains is to minimize the electrostatic energy of depolarizing fields and the elastic energy that is associated with mechanical constraints to which the ferroelectric material is subjected as it is cooled through
TC temperature. The boundary of two neighbouring domains is called a domain wall. In the
absence of an electrical field, the domains are randomly oriented, which result in near complete compensation of the spontaneous polarization. When an external electric field is applied, domains become oriented along the field and the polarization of the material becomes different from zero. The switching of domain orientation proceeds through domain-wall motion [37].
The most prominent feature of a ferroelectric material is the reversibility of the polarization, which results in a hysteresis loop in the dependence of polarization P on electric field E. The polarization increases linearly with the applied electric field, and when the field strength is increased the domains start to align with the direction of the applied field, giving
rise to a rapid non-linear increase in the polarization up to PS. When the external field is
removed, many of the domains are still aligned, hence the material shows a remanent
polarization, Pr. When the electric field with opposite direction is applied, the domain
reorients accordingly, and the external field needed to reduce the polarization to zero is
called the coercive field, EC. Therefore, the (P-E) hysteresis loop is characterized by Ps, Pr,
and EC. Saturation polarization is the maximum polarization that can be reached, remanent
polarization is the polarization present when no electric field is applied, and coercive field is a value of electric filed that is required to bring the polarization to zero [28].
Ferroelectric materials also exhibit piezoelectricity and pyroelectricity.Piezoelectricity
The polarization generated from a mechanical stress is called the direct or generator effect, while the converse or motor effect is associated with the mechanical deformation derived from an applied electric field. The relationships between the strain X, stress σ, electric field strength E, electric polarisation P in a piezoelectric material are [28]:
𝑃 = 𝑑×𝜎 (1)
(direct effect)
𝑋 = 𝑑×𝐸 (2)
(converse effect) where, d is a piezoelectric constant.
In a pyroelectric materials the change in temperature produces a change in spontaneous polarisation [28]. All ferroelectrics belong to pyroelectric and piezoelectric classes, but not vice versa [35]. Figure 2-4 illustrates the basis of piezoelectricity, pyroelectricity and ferroelectricity, as well as the classification of material groups based on the symmetry point group.
Figure 2-4. Schematic representation of piezoelectricity, pyroelectricity and ferroelectricity based on crystal symmetry and their origin. Scheme also shows that all
Because of their high dielectric constant, large polarization, and piezoelectric properties, ferroelectric materials have a wide range of applications, as presented in Figure 2-5. The high dielectric constant of ferroelectrics has been utilized to produce capacitors with tunable capacitance. Pyroelectricity has been utilized for ultrasensitive infrared detectors. On the other hand, piezoelectricity has made materials applicable to high performance actuators, vibration sensors, and other devices. Among the many attractive physical properties of ferroelectric oxides, the reversible spontaneous electric polarization has attracted attention, as a medium for non-volatile data storage devices [38].
Figure 2-5. Some of the examples of applications of ferroelectric materials due to their piezoelectric, dielectric and pyroelectric properties [38]. (Thin Film PZT for Semiconductor Application Trends & Technology 2013 Report by Yole Developpement).
IPD – Integrated Passives Devices, HDD – Hard Disk Drive, MEMS -
MicroElectroMechanical System, FeRAM - Ferroelectric Random Access Memory, IR – Infrared.
Trend towards miniaturisation of electronic devices has been the driving force for the reduction of the size of the ferroelectric material. Especially in the form of thin films, ferroelectrics promise cost effective devices with an important functionality, high performance and a low energy consumption. Typical device examples include thin films for non-volatile ferroelectric random access memories (FeRAM), microactuators, microwave phase shifters, infrared sensors and ferroelectric memristors. Very recent review on thin film ferroelectric materials and their applications discusses several exciting possibilities for the development of new devices, including those in electronic, thermal and photovoltaic applications, and transduction sensors and actuators [39]. Currently the fabrication of those devices involves the integration of the ferroelectric thin film with the Complementary Metal Oxide Semiconductor (CMOS) Integrated Circuit (IC). Therefore, the technologies for fabricating ferroelectric films on silicon have developed rapidly during last years. Figure 2-6 summarizes the most important thin films deposition techniques.
